Thyroid hormones may play an important role in the growth and development of the chick embryo. Their direct effects on chick cartilage maturation, metabolism and sulfation have been reported. Somatomedin is also known to enhance the sulfation of chick cartilage. The proposed research will investigate the relationship of somatomedin and the direct action of thyroid hormones on the sulfation of chick cartilage. It will also determine the somatomedin concentration of chick embryonic serum. It is also known that the synthesis of chondroitin sulfate isomers changes with the stages of chick embryo development. The ratio of the synthesis of Ch-4-SO4 to the synthesis of Ch-6-SO4 in epiphyseal cartilage increases with the age of the embryo. We will evaluate the effects of thyroid hormones on the extent of sulfation and the proportion of the chondroitin sulfate isomers in various stages of development. Radioimmunoassays of thyroxine and triiodothyronine will be used to evaluate the thyroid function of the chick embryo. The metabolic status of the embryos will be evaluated using basal metabolic rates and liver and cartilage oxygen consumption and succinic dehydrogenase activity.